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During emersion, diatoms are exposed to sunlight (with UVB) on tidal flats, where the salinity may increase as a result of evaporation. In order to investigate whether a combination of UV radiation and desiccation would cause oxidative stress under such conditions, an experiment has been carried out with the diatom Cylindrotheca closterium. Cell division rates, photosynthetic efficiencies, pigment contents, and activities of antioxidant enzymes (SOD, APX, MDHAR, GR) were analysed in exponential-phase batch cultures grown at 35 (normal salinity) and 70 PSU (hyperosmotic) in a 12 h:12 h light:dark cycle (268 µmol photons m-2 s-1 photosynthetically active radiation; PAR) at 15°C. Within the 12 h photoperiod, UVA (2.80 W m-2 unweighted) or UVA+UVB (4.23 and 0.22 W m-2, respectively) were supplemented during 4 h. Separately as well as in combination, UVB (3.45 kJ m-2 d-1) and salt stress (70 PSU) caused a decrease in division rates. This UVB dose caused a decrease in photosynthetic efficiency, whereas salt stress did not. Cell volumes of UVB-exposed C. closterium increased only at 70 PSU. UVB radiation and salt stress caused significant decreases in chl a, chl c and fucoxanthin contents, but the UVB effect was stronger than the salt effect. The relatively high b-carotene:chl a ratios in UVB-exposed and salt-stressed cells might indicate that b-carotene was used in scavenging singlet oxygen. In high light with UVA (at 35 PSU), SOD, APX, MDHAR and GR activities were not higher than in low light (27 µmol photons m-2 s-1). Separately and in combination, UVB and salt stress enhanced SOD activity in C. closterium, whereas APX was stimulated by UVB only. MDHAR was stimulated under UVB and salt stress, but there were no effects on GR activity. Under ambient salt and UV conditions during emersion, oxidative stress may contribute to an inhibition of growth of C. closterium [KEYWORDS: Antioxidants; Cylindrotheca closterium; Diatom; Pigments; Salt stress; Ultraviolet]